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Fix potential race conditions in caching

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Use a 'set to None' sentinel to indicate data updates are in progress and we
need to hold off a bit on caching a new value. This logic is gleamed from
the "Scaling Django" slides presented by Mike Malone and available freely on
SlideShare.

Signed-off-by: Dan McGee <dan@archlinux.org>
This commit is contained in:
Dan McGee 2010-10-08 17:09:34 -05:00
parent 1946d3d144
commit 16b27bc9e1
1 changed files with 14 additions and 14 deletions
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28
feeds.py
View File

@ -12,6 +12,8 @@
from news.models import News
CACHE_TIMEOUT = 1800
INVALIDATE_TIMEOUT = 15
CACHE_PACKAGE_KEY = 'cache_package_latest'
CACHE_NEWS_KEY = 'cache_news_latest'
@ -36,7 +38,10 @@ def retrieve_package_latest():
latest = Package.objects.values('last_update').latest(
'last_update')['last_update']
latest = latest + utc_offset()
cache.set(CACHE_PACKAGE_KEY, latest, CACHE_TIMEOUT)
# Using add means "don't overwrite anything in there". What could be in
# there is an explicit None value that our refresh signal set, which
# means we want to avoid race condition possibilities for a bit.
cache.add(CACHE_PACKAGE_KEY, latest, CACHE_TIMEOUT)
return latest
except Package.DoesNotExist:
pass
@ -45,11 +50,10 @@ def retrieve_package_latest():
def refresh_package_latest(**kwargs):
# We could delete the value, but that could open a race condition
# where the new data wouldn't have been committed yet by the calling
# thread. Update it instead.
latest = Package.objects.values('last_update').latest(
'last_update')['last_update']
latest = latest + utc_offset()
cache.set(CACHE_PACKAGE_KEY, latest, CACHE_TIMEOUT)
# thread. Instead, explicitly set it to None for a short amount of time.
# Hopefully by the time it expires we will have committed, and the cache
# will be valid again. See "Scaling Django" by Mike Malone, slide 30.
cache.set(CACHE_PACKAGE_KEY, None, INVALIDATE_TIMEOUT)
def package_etag(request, *args, **kwargs):
latest = retrieve_package_latest()
@ -123,20 +127,16 @@ def retrieve_news_latest():
latest = News.objects.values('last_modified').latest(
'last_modified')['last_modified']
latest = latest + utc_offset()
cache.set(CACHE_NEWS_KEY, latest, CACHE_TIMEOUT)
# same thoughts apply as in retrieve_package_latest
cache.add(CACHE_NEWS_KEY, latest, CACHE_TIMEOUT)
return latest
except News.DoesNotExist:
pass
return None
def refresh_news_latest(**kwargs):
# We could delete the value, but that could open a race condition
# where the new data wouldn't have been committed yet by the calling
# thread. Update it instead.
latest = News.objects.values('last_modified').latest(
'last_modified')['last_modified']
latest = latest + utc_offset()
cache.set(CACHE_NEWS_KEY, latest, CACHE_TIMEOUT)
# same thoughts apply as in refresh_package_latest
cache.set(CACHE_NEWS_KEY, None, INVALIDATE_TIMEOUT)
def news_etag(request, *args, **kwargs):
latest = retrieve_news_latest()